Eyeing China, Biden and allies unveil nuclear-powered submarine plan for Australia - Reuters

The plan "constitutes serious nuclear proliferation risks, undermines international non-proliferation system, fuels arms races, and hurts peace and stability," China's permanent mission to the United Nations said in a tweet after the announcement.

U.S. national security adviser Jake Sullivan pointed on Friday to Beijing's own military buildup, including nuclear-powered submarines, saying: "We have communicated with them about AUKUS and sought more information from them about their intentions."

Big questions remain about AUKUS, not least over strict U.S. curbs on the extensive technology sharing needed for the project and about how long it will take to deliver the submarines, even as the perceived threat posed by China mounts.

In a reflection of stretched U.S. production capacity, a second senior U.S. official told Reuters it was "very likely" one or two of the Virginia-class submarines sold to Australia would be vessels that had been in U.S. service, something that would require congressional approval.

Analysts said that given China's growing power and threats to take Taiwan by force if necessary, it was vital to advance the second stage of AUKUS, which involves hypersonics and other weaponry that can be deployed more quickly.

British and Australian officials said this month work was still needed to break down bureaucratic barriers to technology sharing and Monday's announcement did not cover this second stage.